randomized 2d array

Hi everyone!
I need help with a 2d array that's 80x20 and has 100 elements placed randomly in it, the other elements are blank spaces so it'll look like a starry sky.
I have a preliminary code figured out, but I need a mechanism that'll set 100 "*" elements randomly in the array. I was thinking maybe an if else statement would work, but am at a loss as to how that should be done.
If someone could point me in the right direction that would be magnificent!